Process and composition for chemically treating titanium and its alloys



United States Patent PROCESS AND COMPOSITION FOR CHEMICALLY TREATINGTITANIUM AND ITS ALLOYS Albert Borowik, Bridgeport, Pa., assignor to theUnited States of America as represented by the Secretary of the Army NoDrawing. Filed Feb. 8, 1957, Ser. No. 639,121

8 Claims. (Cl. 41-42) (Granted under Title 35, U.S. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes without the payment of anyroyalty thereon.

This invention relates to a material and process for the chemicalpolishing and etching of titanium and its alloys for metallographicexamination and has for an object to provide a suitable reagent andprocess for this purpose.

An examination of the true microstructure of a metal specimen has beensaid to be the prime function of chemical as distinguished fromelectroyltic polishing. The specimen may have been mechanically polishedbeforehand to obtain good light-reflective surface. Chemical etchingdissolves more metal at grain boundaries than from the surface of thegrains in order to provide better grain delineation.

Titanium and its solid solution alloys have been diflicult to polishmechanically because the metal is quickly work-hardened yet at the sametime it has a tendency to flow and hide the true grain structure in amanner sowewhat analogous to the way in which a plastic will be deformedand hide a subsurface layer. Hand-polishing of such metal has been donebut such is tedious for titanium and its alloys for the reasonmentioned.

According to this invention a reagent has been discovered which ispeculiarly suited to give good results with metallographic specimen oftitanium and its alloys. The reasons therefor are unknown and thediscovery of such has been empirical. Specifically, only one reagentformed from a number of old ingredients has been found satisfactory forthe intended purpose. That one reagent includes 10 cc. of 48% GP.hydrofluoric acid with 10 cc. of 69.3% of commercial nitric acid, 60 cc.of U.S.P. 3% hydrogen peroxide, and 30 cc. of distilled water. A betterreagent composition has been found to result from the addition to theforegoing of 150 cc. of commercial glycerol having a specific gravity of1.249. The best reagent for titanium metals and alloys is when 300 cc.of commercial ethylene glycol is added to the first mentionedcombination of ingredients. In that first combination, plus or minus 5%by volume of each acid, the hydrogen peroxide and the distilled waterconstitute approximate limits for each of those components.

The time of immersion may vary depending upon the results desired, fromto 60 seconds. The Work is then rinsed and dried. Having the polishingand etching bath at a slightly elevated temperature but not over about200 F. will reduce the immersion time.

The functions of the various ingredients is not certain. Thehydrofluoric acid is possibly the principal acid for attack upon themetal. The nitric acid and the hydrogen peroxide both cooperate in anoxidizing reaction. The distilled water serves as both a diluent and asa carrier or vehicle in producing a uniformly distributed polishingaction. The ethylene glycol is believed to function as a carrier toinsure uniformity in distribution of the polishing and etching action.The same may be said of the glycerol. Whatever the actions of theseingredients, it should be emphasized that their cooperative actionresults in the most satisfactory results with titanium solid solutionalloys.

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Among the advantages of this invention may be mentioned the provision ofa polished light reflecting surface of titanium alloys at the same timea revealing etched surface is provided whereby the previous diflicultieswith mechanical polishing of this metal have been eliminated.

I claim:

1. A metallographic reagent for chemical polishing and simultaneouslypreferentially etching metallographic specimen of titanium and itsalloys in solid solution whereby they are made ready for microscopicmetallographic examination, said reagent consisting essentially of 10parts by volume of 48% GP. hydrofluoric acgl, 10 parts by volume ofcommercial 69.3% nitric acid, 60 parts by volume of U.S.P. 3% hydrogenperoxide and 30 parts by volume of distilled water.

2. A chemical polishing reagent according to claim 1 having also atleast and not more than 300 parts by volume of one from a groupconsisting of ethylene glycol and glycerol.

3. A chemical polishing reagent according to claim 2 containing 150parts by volume of glycerol.

4. A chemical polishing reagent according to claim 2 containing 300parts by volume of ethylene glycol.

5. A process for preparing a specimen of titanium solid solution alloysfor metallographic examination by reflected light from a specimen into amicroscope which comprises chemically polishing said specimen to enhancereflectivity of light rays incident upon a surface of the specimen intoa microscope, and simultaneously etching said surface of said specimenfor delineation of grain boundaries during said polishing by reactinggrain boundaries in said surface to the action of a reagent at atemperature of not over about 200 F. for not more than about one minutedepending on the temperature of the reagent, and terminating saidreaction by separating said surface from said reagent, when said reagentconsists essentially of 10 parts by volume of 48% GP. hydrofluoric acid,60 parts by volume of U.S.P. 3% hydrogen peroxide, 10 parts by volume ofcommercial 69.3% nitric acid, and 30 parts by volume of distilled water.

6. A process according to claim 5 which includes in said reagent atleast about 150 parts by volume and not more than about 300 parts byvolume of one member of a group consisting of glycerol and ethyleneglycol.

7. A process according to claim 5 in which said reagent includes about150 parts by volume of glycerol.

8. A process according to claim 5 in which said reagent includes about300 parts by volume of ethylene glycol.

References Cited in the tile of this patent UNITED STATES PATENTS2,542,727 Theuerer Feb. 20, 1951 2,606,960 Little Aug. 12, 19522,687,345 Murray Aug. 24, 1954 2,711,364 Beach June 21, 1955 2,724,667MacPherson Nov. 22, 1955 2,734,837 Hands Feb. 14, 1956 2,828,192Langfield Mar. 25, 1958 2,876,144 Bomberger Mar. 3, 1959 FOREIGN PATENTS753,158 Great Britain July 18, 1956 789,625 Great Britain Ian. 22, 1958OTHER REFERENCES New Etches for Germanium, by Ellis and Wolsky,

from the Journal of Applied Physics, vol. 24, 1953; pages 1411 and 1412.

Lflhi'simati

1. A METALLOGRAPHIC REAGENT FOR CHEMICAL POLISHING AND SIMULTANEOUSLYPREFERENTIALLY ETCHING METALLOGRAPHIC SPECIMEN OF TITANIUM AND ITSALLOYS IN SOLID SOLUTION WHEREBY THEY ARE MADE READY FOR MICROSCOPICMETALLOGRAPHIC EXAMINATION, SAID REAGENT CONSISTING ESSENTIALLY OF 10PARTS BY VOLUME OF 48% C.P. HYDROFLUORIC ACID, 10 PARTS BY VOLUME OFCOMMERCIAL 69.3% NITRIC ACID, 60 PARTS BY VOLUME OF U.S.P. 3% HYDROGENPEROXIDE AND 30 PARTS BY VOLUME OF DISTILLED WATER.
 5. A PROCESS FORPREPARING A SPECIMEN OF TITANIUM SOLID SOLUTION ALLOYS FORMETALLOGRAPHIC EXAMINATION BY REFLECTED LIGHT FROM A SPECIMEN INTO AMICROSCOPE WHICH COMPRISES CHEMICALLY POLISHING SAID SPECIMEN TO ENHANCEREFLECTIVITY OF LIGHT RAYS INCIDENT UPON A SURFACE OF THE SPECIMEN INTOA MICROSCOPE, AND SIMULTANEOUSLY ETCHING SAID SURFACE OF SAID SPECIMENFOR DELINEATION OF GRAIN BOUNDARIES DURING SAID POLISHING BY REACTINGGRAIN BOUNDARIES IN SAID SURFACE TO THE ACTION OF A REAGENT AT ATEMPERATURE OF NOT OVER ABOUT 200* F. FOR NOT MORE THAN ABOUT ONE MINUTEDEPENDING ON THE TEMPERATURE OF THE REAGENT, AND TERMINATING SAIDREACTION BY SEPARATING SAID SURFACE FROM SAID REAGENT, WHEN SAID REAGENTCONSISTS ESSENTIALLY OF 10 PARTS BY VOLUME OF 48% C.P. HYDROFLUORICACID, 60 PARTS BY VOLUME OF U.S.P. 3% HYDROGEN PEROXIDE, 10 PARTS BYVOLUME OF COMMERCIAL 69.3% NITRIC ACID, AND 30 PARTS BY VOLUME OFDISTILLED WATER.